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Chapter 24
Database
Security
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
1 Introduction to Database Security
Issues

Types of Security




Legal and ethical issues
Policy issues ISO 27001 :2013
System-related issues
The need to identify multiple security levels
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
Introduction to Database Security Issues
(2)

Threats to databases
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Loss of integrity
Loss of availability
Loss of confidentiality
To protect databases against these types of threats four
kinds of countermeasures can be implemented:
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Access control
Inference control
Flow control
Encryption
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
Introduction to Database
Security Issues (3)

A DBMS typically includes a database security
and authorization subsystem that is responsible
for ensuring the security portions of a database
against unauthorized access.

Two types of database security mechanisms:
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Discretionary security mechanisms
Mandatory security mechanisms
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
Introduction to Database
Security Issues (4)

The security mechanism of a DBMS must include
provisions for restricting access to the database
as a whole

This function is called access control and is
handled by creating user accounts and passwords
to control login process by the DBMS.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
Introduction to Database
Security Issues (5)

The security problem associated with databases
is that of controlling the access to a statistical
database, which is used to provide statistical
information or summaries of values based on
various criteria.

The countermeasures to statistical database
security problem is called inference control
measures.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
Introduction to Database
Security Issues (6)

Another security is that of flow control, which
prevents information from flowing in such a way
that it reaches unauthorized users.

Channels that are pathways for information to
flow implicitly in ways that violate the security
policy of an organization are called covert
channels.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
Introduction to Database
Security Issues (7)

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A final security issue is data encryption, which is
used to protect sensitive data (such as credit card
numbers) that is being transmitted via some type
communication network.
The data is encoded using some encoding
algorithm.

An unauthorized user who access encoded data
will have difficulty deciphering it, but authorized
users are given decoding or decrypting algorithms
(or keys) to decipher data.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
1.2 Database Security and the DBA
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The database administrator (DBA) is the central
authority for managing a database system.
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The DBA’s responsibilities include
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granting privileges to users who need to use the
system
classifying users and data in accordance with the
policy of the organization
The DBA is responsible for the overall security of
the database system.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
1.2 Database Security and the DBA (2)
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The DBA has a DBA account in the DBMS
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Sometimes these are called a system or superuser account
These accounts provide powerful capabilities such as:
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1. Account creation
2. Privilege granting
3. Privilege revocation
4. Security level assignment
Action 1 is access control, whereas 2 and 3 are
discretionarym and 4 is used to control mandatory
authorization
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
1.3 Access Protection, User Accounts,
and Database Audits

Whenever a person or group of person s need to
access a database system, the individual or
group must first apply for a user account.
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The DBA will then create a new account id and
password for the user if he/she deems there is a
legitimate need to access the database
The user must log in to the DBMS by entering
account id and password whenever database
access is needed.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
1.3 Access Protection, User Accounts,
and Database Audits(2)
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The database system must also keep track of all
operations on the database that are applied by a
certain user throughout each login session.

To keep a record of all updates applied to the
database and of the particular user who applied
each update, we can modify system log, which
includes an entry for each operation applied to the
database that may be required for recovery from a
transaction failure or system crash.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
1.3 Access Protection, User Accounts,
and Database Audits(3)
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If any tampering with the database is suspected,
a database audit is performed
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A database audit consists of reviewing the log to
examine all accesses and operations applied to
the database during a certain time period.
A database log that is used mainly for security
purposes is sometimes called an audit trail.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
Discretionary Access Control Based
on Granting and Revoking Privileges

The typical method of enforcing discretionary
access control in a database system is based
on the granting and revoking privileges.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
2.1Types of Discretionary Privileges
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The account level:
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At this level, the DBA specifies the particular
privileges that each account holds independently
of the relations in the database.
The relation level (or table level):

At this level, the DBA can control the privilege to
access each individual relation or view in the
database.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
2.1Types of Discretionary Privileges(2)

The privileges at the account level apply to the
capabilities provided to the account itself and can include
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the CREATE SCHEMA or CREATE TABLE privilege, to
create a schema or base relation;
the CREATE VIEW privilege;
the ALTER privilege, to apply schema changes such adding
or removing attributes from relations;
the DROP privilege, to delete relations or views;
the MODIFY privilege, to insert, delete, or update tuples;
and the SELECT privilege, to retrieve information from the
database by using a SELECT query.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
2.1Types of Discretionary Privileges(3)

The second level of privileges applies to the relation
level
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This includes base relations and virtual (view) relations.
The granting and revoking of privileges generally follow
an authorization model for discretionary privileges known
as the access matrix model where
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The rows of a matrix M represents subjects (users,
accounts, programs)
The columns represent objects (relations, records,
columns, views, operations).
Each position M(i,j) in the matrix represents the types of
privileges (read, write, update) that subject i holds on
object j.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
2.1Types of Discretionary Privileges(4)

To control the granting and revoking of relation privileges,
each relation R in a database is assigned and owner
account, which is typically the account that was used
when the relation was created in the first place.
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The owner of a relation is given all privileges on that
relation.
In SQL2, the DBA can assign and owner to a whole schema
by creating the schema and associating the appropriate
authorization identifier with that schema, using the CREATE
SCHEMA command.
The owner account holder can pass privileges on any of
the owned relation to other users by granting privileges to
their accounts.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
2.1Types of Discretionary Privileges(5)

In SQL the following types of privileges can be granted on
each individual relation R:
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SELECT (retrieval or read) privilege on R:
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Gives the account retrieval privilege.
In SQL this gives the account the privilege to use the SELECT
statement to retrieve tuples from R.
MODIFY privileges on R:
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This gives the account the capability to modify tuples of R.
In SQL this privilege is further divided into UPDATE, DELETE,
and INSERT privileges to apply the corresponding SQL
command to R.
In addition, both the INSERT and UPDATE privileges can
specify that only certain attributes can be updated by the
account.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
2.1Types of Discretionary Privileges(6)

In SQL the following types of privileges can be
granted on each individual relation R (contd.):
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REFERENCES privilege on R:
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This gives the account the capability to reference
relation R when specifying integrity constraints.
The privilege can also be restricted to specific
attributes of R.
Notice that to create a view, the account must
have SELECT privilege on all relations involved
in the view definition.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
2.2 Specifying Privileges Using Views

The mechanism of views is an important discretionary
authorization mechanism in its own right. For example,
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If the owner A of a relation R wants another account B to be
able to retrieve only some fields of R, then A can create a
view V of R that includes only those attributes and then
grant SELECT on V to B.
The same applies to limiting B to retrieving only certain
tuples of R; a view V’ can be created by defining the view by
means of a query that selects only those tuples from R that
A wants to allow B to access.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
2.3 Revoking Privileges

In some cases it is desirable to grant a privilege
to a user temporarily. For example,
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The owner of a relation may want to grant the
SELECT privilege to a user for a specific task and
then revoke that privilege once the task is
completed.
Hence, a mechanism for revoking privileges is
needed. In SQL, a REVOKE command is included
for the purpose of canceling privileges.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
2.4 Propagation of Privileges using the
GRANT OPTION
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Whenever the owner A of a relation R grants a privilege
on R to another account B, privilege can be given to B
with or without the GRANT OPTION.
If the GRANT OPTION is given, this means that B can
also grant that privilege on R to other accounts.
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Suppose that B is given the GRANT OPTION by A and that
B then grants the privilege on R to a third account C, also
with GRANT OPTION. In this way, privileges on R can
propagate to other accounts without the knowledge of the
owner of R.
If the owner account A now revokes the privilege granted to
B, all the privileges that B propagated based on that
privilege should automatically be revoked by the system.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
2.5 An Example

Suppose that the DBA creates four accounts
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A1, A2, A3, A4
and wants only A1 to be able to create base relations.
Then the DBA must issue the following GRANT command
in SQL
GRANT CREATETAB TO A1;

In SQL2 the same effect can be accomplished by having
the DBA issue a CREATE SCHEMA command as
follows:
CREATE SCHAMA EXAMPLE AUTHORIZATION A1;
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
2.5 An Example (2)
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
User account A1 can create tables under the schema
called EXAMPLE.
Suppose that A1 creates the two base relations
EMPLOYEE and DEPARTMENT
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A1 is then owner of these two relations and hence all the
relation privileges on each of them.
Suppose that A1 wants to grant A2 the privilege to insert
and delete tuples in both of these relations, but A1 does
not want A2 to be able to propagate these privileges to
additional accounts:
GRANT INSERT, DELETE ON
EMPLOYEE, DEPARTMENT TO A2;
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
2.5 An Example (3)
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
2.5 An Example (4)
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Suppose that A1 wants to allow A3 to retrieve information
from either of the two tables and also to be able to
propagate the SELECT privilege to other accounts.
A1 can issue the command:
GRANT SELECT ON EMPLOYEE, DEPARTMENT
TO A3 WITH GRANT OPTION;

A3 can grant the SELECT privilege on the EMPLOYEE
relation to A4 by issuing:
GRANT SELECT ON EMPLOYEE TO A4;

Notice that A4 can’t propagate the SELECT privilege
because GRANT OPTION was not given to A4
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
2.5 An Example (5)

Suppose that A1 decides to revoke the SELECT
privilege on the EMPLOYEE relation from A3; A1
can issue:
REVOKE SELECT ON EMPLOYEE FROM A3;

The DBMS must now automatically revoke the
SELECT privilege on EMPLOYEE from A4, too,
because A3 granted that privilege to A4 and A3
does not have the privilege any more.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
2.5 An Example (6)
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Suppose that A1 wants to give back to A3 a limited capability to
SELECT from the EMPLOYEE relation and wants to allow A3 to be
able to propagate the privilege.
 The limitation is to retrieve only the NAME, BDATE, and
ADDRESS attributes and only for the tuples with DNO=5.
A1 then create the view:
CREATE VIEW A3EMPLOYEE AS
SELECT NAME, BDATE, ADDRESS
FROM EMPLOYEE
WHERE DNO = 5;

After the view is created, A1 can grant SELECT on the view
A3EMPLOYEE to A3 as follows:
GRANT SELECT ON A3EMPLOYEE TO A3
WITH GRANT OPTION;
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
2.5 An Example (7)


Finally, suppose that A1 wants to allow A4 to update only
the SALARY attribute of EMPLOYEE;
A1 can issue:
GRANT UPDATE ON EMPLOYEE (SALARY) TO
A4;
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The UPDATE or INSERT privilege can specify particular
attributes that may be updated or inserted in a relation.
Other privileges (SELECT, DELETE) are not attribute
specific.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
2.6 Specifying Limits on Propagation of
Privileges

Techniques to limit the propagation of privileges
have been developed, although they have not yet
been implemented in most DBMSs and are not a
part of SQL.
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Limiting horizontal propagation to an integer
number i means that an account B given the
GRANT OPTION can grant the privilege to at most
i other accounts.
Vertical propagation is more complicated; it limits
the depth of the granting of privileges.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
3 Mandatory Access Control and Role-Based
Access Control for Multilevel Security
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The discretionary access control techniques of granting
and revoking privileges on relations has traditionally been
the main security mechanism for relational database
systems.
This is an all-or-nothing method:
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A user either has or does not have a certain privilege.
In many applications, and additional security policy is
needed that classifies data and users based on security
classes.

This approach as mandatory access control, would
typically be combined with the discretionary access control
mechanisms.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
3 Mandatory Access Control and Role-Based
Access Control for Multilevel Security (2)

Typical security classes are top secret (TS), secret (S),
confidential (C), and unclassified (U), where TS is the
highest level and U the lowest: TS ≥ S ≥ C ≥ U

The commonly used model for multilevel security, known
as the Bell-LaPadula model, classifies each subject
(user, account, program) and object (relation, tuple,
column, view, operation) into one of the security
classifications, T, S, C, or U:

Clearance (classification) of a subject S as class(S) and to
the classification of an object O as class(O).
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
3 Mandatory Access Control and Role-Based
Access Control for Multilevel Security (3)

Two restrictions are enforced on data access
based on the subject/object classifications:
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Simple security property: A subject S is not
allowed read access to an object O unless
class(S) ≥ class(O).
A subject S is not allowed to write an object O
unless class(S) ≤ class(O). This known as the star
property (or * property).
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
3 Mandatory Access Control and Role-Based
Access Control for Multilevel Security (4)
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To incorporate multilevel security notions into the relational database
model, it is common to consider attribute values and tuples as data
objects.
Hence, each attribute A is associated with a classification attribute
C in the schema, and each attribute value in a tuple is associated with
a corresponding security classification.
In addition, in some models, a tuple classification attribute TC is
added to the relation attributes to provide a classification for each
tuple as a whole.
Hence, a multilevel relation schema R with n attributes would be
represented as
 R(A1,C1,A2,C2, …, An,Cn,TC)
where each Ci represents the classification attribute associated with
attribute Ai.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
3 Mandatory Access Control and Role-Based
Access Control for Multilevel Security (5)

The value of the TC attribute in each tuple t –
which is the highest of all attribute classification
values within t – provides a general classification
for the tuple itself, whereas each Ci provides a
finer security classification for each attribute value
within the tuple.

The apparent key of a multilevel relation is the set
of attributes that would have formed the primary
key in a regular (single-level) relation.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
3 Mandatory Access Control and Role-Based
Access Control for Multilevel Security (6)

A multilevel relation will appear to contain different data to
subjects (users) with different clearance levels.
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In some cases, it is possible to store a single tuple in the
relation at a higher classification level and produce the
corresponding tuples at a lower-level classification through
a process known as filtering.
In other cases, it is necessary to store two or more tuples at
different classification levels with the same value for the
apparent key.
This leads to the concept of polyinstantiation where
several tuples can have the same apparent key value but
have different attribute values for users at different
classification levels.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
3 Mandatory Access Control and Role-Based
Access Control for Multilevel Security (7)
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In general, the entity integrity rule for multilevel
relations states that all attributes that are
members of the apparent key must not be null
and must have the same security classification
within each individual tuple.
In addition, all other attribute values in the tuple
must have a security classification greater than or
equal to that of the apparent key.

This constraint ensures that a user can see the
key if the user is permitted to see any part of the
tuple at all.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
3 Mandatory Access Control and Role-Based
Access Control for Multilevel Security (8)

Other integrity rules, called null integrity and
interinstance integrity, informally ensure that if
a tuple value at some security level can be
filtered (derived) from a higher-classified tuple,
then it is sufficient to store the higher-classified
tuple in the multilevel relation.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
3.1 Comparing Discretionary Access
Control and Mandatory Access Control

Discretionary Access Control (DAC) policies
are characterized by a high degree of flexibility,
which makes them suitable for a large variety of
application domains.

The main drawback of DAC models is their
vulnerability to malicious attacks, such as Trojan
horses embedded in application programs.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
3.1 Comparing Discretionary Access
Control and Mandatory Access Control(2)
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By contrast, mandatory policies ensure a high
degree of protection in a way, they prevent any
illegal flow of information.
Mandatory policies have the drawback of being
too rigid and they are only applicable in limited
environments.
In many practical situations, discretionary policies
are preferred because they offer a better trade-off
between security and applicability.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
3.2 Role-Based Access Control
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Role-based access control (RBAC) emerged rapidly in
the 1990s as a proven technology for managing and
enforcing security in large-scale enterprisewide systems.
Its basic notion is that permissions are associated with
roles, and users are assigned to appropriate roles.
Roles can be created using the CREATE ROLE and
DESTROY ROLE commands.

The GRANT and REVOKE commands discussed under
DAC can then be used to assign and revoke privileges from
roles.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
3.2 Role-Based Access Control (2)
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RBAC appears to be a viable alternative to
traditional discretionary and mandatory access
controls; it ensures that only authorized users are
given access to certain data or resources.
Many DBMSs have allowed the concept of roles,
where privileges can be assigned to roles.
Role hierarchy in RBAC is a natural way of
organizing roles to reflect the organization’s lines
of authority and responsibility.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
3.2 Role-Based Access Control (3)
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Another important consideration in RBAC systems is the
possible temporal constraints that may exist on roles,
such as time and duration of role activations, and timed
triggering of a role by an activation of another role.
Using an RBAC model is highly desirable goal for
addressing the key security requirements of Web-based
applications.
In contrast, discretionary access control (DAC) and
mandatory access control (MAC) models lack
capabilities needed to support the security requirements
emerging enterprises and Web-based applications.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
3.3 Access Control Policies for
E-Commerce and the Web

E-Commerce environments require elaborate
policies that go beyond traditional DBMSs.
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In an e-commerce environment the resources to
be protected are not only traditional data but also
knowledge and experience.
The access control mechanism should be flexible
enough to support a wide spectrum of
heterogeneous protection objects.
A related requirement is the support for contentbased access-control.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
3.3 Access Control Policies for
E-Commerce and the Web (2)

Another requirement is related to the heterogeneity of
subjects, which requires access control policies based on
user characteristics and qualifications.
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A possible solution, to better take into account user profiles
in the formulation of access control policies, is to support
the notion of credentials.
A credential is a set of properties concerning a user that
are relevant for security purposes

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For example, age, position within an organization
It is believed that the XML language can play a key role in
access control for e-commerce applications.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
4 Introduction to Statistical
Database Security
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
Statistical databases are used mainly to
produce statistics on various populations.
The database may contain confidential data on
individuals, which should be protected from user
access.
Users are permitted to retrieve statistical
information on the populations, such as
averages, sums, counts, maximums,
minimums, and standard deviations.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
4 Introduction to Statistical
Database Security (2)


A population is a set of tuples of a relation
(table) that satisfy some selection condition.
Statistical queries involve applying statistical
functions to a population of tuples.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
4 Introduction to Statistical
Database Security (3)

For example, we may want to retrieve the number of
individuals in a population or the average income in the
population.
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However, statistical users are not allowed to retrieve
individual data, such as the income of a specific person.
Statistical database security techniques must prohibit the
retrieval of individual data.
This can be achieved by prohibiting queries that retrieve
attribute values and by allowing only queries that involve
statistical aggregate functions such as COUNT, SUM,
MIN, MAX, AVERAGE, and STANDARD DEVIATION.

Such queries are sometimes called statistical queries.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
4 Introduction to Statistical
Database Security (4)


It is DBMS’s responsibility to ensure confidentiality of
information about individuals, while still providing useful
statistical summaries of data about those individuals to
users. Provision of privacy protection of users in a
statistical database is paramount.
In some cases it is possible to infer the values of
individual tuples from a sequence statistical queries.

This is particularly true when the conditions result in a
population consisting of a small number of tuples.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
5 Introduction to Flow Control


Flow control regulates the distribution or flow of
information among accessible objects.
A flow between object X and object Y occurs when a
program reads values from X and writes values into Y.
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Flow controls check that information contained in some
objects does not flow explicitly or implicitly into less
protected objects.
A flow policy specifies the channels along which
information is allowed to move.

The simplest flow policy specifies just two classes of
information:
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
confidential (C) and nonconfidential (N)
and allows all flows except those from class C to class N.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
5.1 Covert Channels

A covert channel allows a transfer of information
that violates the security or the policy.

A covert channel allows information to pass
from a higher classification level to a lower
classification level through improper means.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
5.1 Covert Channels (2)

Covert channels can be classified into two broad
categories:
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Storage channels do not require any temporal
synchronization, in that information is conveyed by
accessing system information or what is otherwise
inaccessible to the user.
Timing channel allow the information to be conveyed by
the timing of events or processes.
Some security experts believe that one way to avoid
covert channels is for programmers to not actually gain
access to sensitive data that a program is supposed to
process after the program has been put into operation.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
6 Encryption and Public Key
Infrastructures

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
Encryption is a means of maintaining secure
data in an insecure environment.
Encryption consists of applying an encryption
algorithm to data using some prespecified
encryption key.
The resulting data has to be decrypted using a
decryption key to recover the original data.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
6.1 The Data and Advanced Encryption
Standards

The Data Encryption Standard (DES) is a
system developed by the U.S. government for
use by the general public.


It has been widely accepted as a cryptographic
standard both in the United States and abroad.
DES can provide end-to-end encryption on the
channel between the sender A and receiver B.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
6.1 The Data and Advanced Encryption
Standards (2)

DES algorithm is a careful and complex
combination of two of the fundamental building
blocks of encryption:


substitution and permutation (transposition).
The DES algorithm derives its strength from
repeated application of these two techniques for
a total of 16 cycles.

Plaintext (the original form of the message) is
encrypted as blocks of 64 bits.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
6.1 The Data and Advanced Encryption
Standards(3)

After questioning the adequacy of DES, the
National Institute of Standards (NIST) introduced
the Advanced Encryption Standards (AES).

This algorithm has a block size of 128 bits and
thus takes longer time to crack.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
6.2 Public Key Encryption
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
In 1976 Diffie and Hellman proposed a new kind of
cryptosystem, which they called public key encryption.
Public key algorithms are based on mathematical
functions rather than operations on bit patterns.

They also involve the use of two separate keys
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
in contrast to conventional encryption, which uses only one
key.
The use of two keys can have profound consequences in
the areas of confidentiality, key distribution, and
authentication.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
6.2 Public Key Encryption (2)

The two keys used for public key encryption are
referred to as the public key and the private
key.

the private key is kept secret, but it is referred to
as private key rather than a secret key (the word
used in conventional encryption to avoid confusion
with conventional encryption).
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
6.2 Public Key Encryption (3)

A public key encryption scheme, or infrastructure, has six
ingredients:
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Plaintext: This is the data or readable message that is fed
into the algorithm as input.
Encryption algorithm: The encryption algorithm performs
various transformations on the plaintext.
Public and private keys: These are pair of keys that have
been selected so that if one is used for encryption, the other
is used for decryption.

The exec transformations performed by the encryption
algorithm depend on the public or private key that is provided
as input.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
6.2 Public Key Encryption (4)

A public key encryption scheme, or infrastructure,
has six ingredients (contd.):

Ciphertext:



This is the scrambled message produced as
output. It depends on the plaintext and the key.
For a given message, two different keys will
produce two different ciphertexts.
Decryption algorithm:

This algorithm accepts the ciphertext and the
matching key and produces the original plaintext.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
6.2 Public Key Encryption (5)


Public key is made for public and private key is
known only by owner.
A general-purpose public key cryptographic
algorithm relies on


one key for encryption and
a different but related key for decryption.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
6.2 Public Key Encryption (6)

The essential steps are as follows:




Each user generates a pair of keys to be used for the
encryption and decryption of messages.
Each user places one of the two keys in a public register or
other accessible file. This is the public key. The companion
key is kept private (private key).
If a sender wishes to send a private message to a receiver,
the sender encrypts the message using the receiver’s
public key.
When the receiver receives the message, he or she
decrypts it using the receiver’s private key.

No other recipient can decrypt the message because only the
receiver knows his or her private key.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
6.2 Public Key Encryption (7)

The RSA Public Key Encryption algorithm, one
of the first public key schemes was introduced in
1978 by Ron Rivest (R), Adi Shamir (S), and Len
Adleman (A) at MIT and is named after them.


The RSA encryption algorithm incorporates results
from number theory, such as the difficulty of
determining the large prime factors of a large
number.
The RSA algorithm also operates with modular
arithmetic – mod n, where n is the product of two
large prime numbers.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
6.2 Public Key Encryption (8)

Two keys, d and e, are used for decryption and encryption.
 An important property is that d and e can be interchanged.
 n is chosen as a large integer that is a product of two large
distinct prime numbers, a and b.
 The encryption key e is a randomly chosen number between 1
and n that is relatively prime to (a-1) x (b-1).
 The plaintext block P is encrypted as Pe mod n.
 Because the exponentiation is performed mod n, factoring Pe to
uncover the encrypted plaintext is difficult.
 However, the decryption key d is carefully chosen so that
(Pe)d mod n = P.
 The decryption key d can be computed from the condition that
d x e= 1 mod ((a-1)x(b-1)).
 Thus, the legitimate receiver who knows d simply computes
(Pe)d mod n = P and recovers P without having to factor Pe .
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
6.3 Digital Signatures



A digital signature is an example of using encryption techniques to
provide authentication services in e-commerce applications.
A digital signature is a means of associating a mark unique to an
individual with a body of text.
 The mark should be unforgettable, meaning that others should be
able to check that the signature does come from the originator.
A digital signature consists of a string of symbols.
 Signature must be different for each use.


This can be achieved by making each digital signature a function of
the message that it is signing, together with a time stamp.
Public key techniques are the means creating digital signatures.
Copyright © 2011 Ramez Elmasri and Shamkant Navathe
Summary






1 Database Security and Authorization
2 Discretionary Access Control
3 Mandatory Access Control and Role-Based
Access Control for Multilevel Security
4 Statistical Database Security
5 Flow Control
6 Encryption and Public Key Infrastructures
Copyright © 2011 Ramez Elmasri and Shamkant Navathe